This is the 27th most frequent Czech word.
být
"To be."
The word "být" is used here as the verb "to be" in the present tense, indicating the state of being at a location.
Já jsem doma.
I am at home.
Here, "být" represents the verb "to be" in the future tense, expressing a planned future state or occupation.
Budu doktor.
I will be a doctor.
In this sentence, "být" is part of a modal construct expressing obligation, combined with "mám" (meaning "I should").